Method for using a mobile device equipped with at least two microphones for determining the direction of loudspeakers in a setup of a surround sound system

ABSTRACT

A smartphone having two microphones is used for determining the direction of a loudspeaker in a surround system setup. This is performed using smartphone rotation in azimuth and polar angle direction while capturing in its microphones a test signal from a current one of the loudspeakers. From the microphone signals a corresponding TDOA value is calculated, and the smartphone is rotated until that TDOA value is nearly zero, resulting in a loudspeaker direction information.

REFERENCE TO RELATED EUROPEAN APPLICATION

This application claims priority from European No. 15307064.4, entitled “Method For Using A Mobile Device Equipped With At Least Two Microphones for Determining The Direction Of Loudspeakers In A Setup Of A Surround Sound System,” filed on Dec. 18, 2015, the contents of which are hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The invention relates to a method for using a mobile device equipped with at least two microphones for determining the direction of loudspeakers l_(k) in a setup of a surround sound system including N loudspeakers, k=1 . . . N.

BACKGROUND

For 3D sound reproduction more and more loudspeakers are required for rendering additional audio channels that surround the listener. To ensure the best listener experience, this implicitly requires the correct determination of the direction as part of the position information of each loudspeaker, in order to accurately calibrate the array of speakers and to ensure a correct rendering process.

Currently different methods are available for determination of the direction of arrival, requiring the use of a multi-microphone device. This results in additional costs at user side.

SUMMARY OF INVENTION

Today the number of smartphones equipped with more than one microphone (two or three) is increasing. A smartphone having at least two microphones is used for determining the direction of a loudspeaker in a surround system setup. The resulting effect is calibration equipment for home theatre setup that is today available in most households.

The advantages of using such mobile devices are:

-   -   cheap solution;     -   an improvement of the calibration setup can be achieved by         updating an app;     -   by using more mobile devices including microphones, the         measurement precision can be increased and the calibration time         can be minimised.

A problem to be solved by the invention is to provide a cheap measurement of loudspeaker positions in a surround sound setup. This problem is solved by the method disclosed in claim 1 or in claim 2.

Advantageous additional embodiments of the invention are disclosed in the respective dependent claims.

In principle, the inventive method is adapted for using a mobile device equipped with at least two microphones for determining the direction of loudspeakers l_(k) in a setup of a surround sound system including N loudspeakers, k=1 . . . N, wherein said direction is expressed by an azimuth angle ϕ_(k) and a polar angle θ_(k), said method including:

-   a) setting initial values for said azimuth angle ϕ_(k) and said     polar angle θ_(k) for loudspeaker l_(k) direction; -   b) in a first loop over mobile device position angle α for the     determination of one of ϕ_(k) and θ_(k), and thereafter in a second     loop over mobile device position angle α for the determination of     the other one of ϕ_(k) and θ_(k): -   c) setting k=1; -   d) in a sub-loop over k: -   e) in a sub-sub-loop over a rotation angle of said mobile device: -   f) causing loudspeaker l_(k) to emit a test signal; -   g) rotating said mobile device and providing for said mobile device     a corresponding measured mobile device rotation angle value α_(k); -   h) capturing corresponding mobile device microphone signals from     said loudspeaker l_(k) test signal; -   i) calculating from said microphone signals a corresponding TDOA     value; -   j) if said TDOA value is not zero or is not smaller than a     predetermined threshold value, returning to step f); -   k) otherwise, calculating a corresponding azimuth ϕ_(k) or polar     θ_(k), respectively, angle value for the position of loudspeaker     l_(k); -   l) incrementing k by ‘1’; -   m) if k≤N, returning to step f); -   n) otherwise, checking whether both of ϕ_(k) and θ_(k) have been     determined, and if not true, returning to step b); -   o) after all positions of said N loudspeakers have been determined,     providing a corresponding set of N pairs of azimuth and polar angle     values ϕ_(k) and θ_(k) for said loudspeakers l_(k) and for all k.     or     for using a mobile device equipped with at least two microphones,     having a known distance from each other, for determining the     direction of loudspeakers l_(k) in a setup of a surround sound     system including N loudspeakers, k=1 . . . N, wherein said direction     is expressed by an azimuth angle ϕ_(k) and a polar angle θ_(k), said     method including: -   a) setting initial values for said azimuth angle ϕ_(k) and said     polar angle θ_(k) for loudspeaker l_(k) direction; -   b) in a first loop over mobile device position angle α for the     determination of one of ϕ_(k) and θ_(k), and thereafter in a second     loop over mobile device position angle α for the determination of     the other one of ϕ_(k) and θ_(k): -   c) positioning said mobile device at a desired azimuth angle or     polar angle; -   d) setting k=1; -   e) in a sub-loop over k: -   f) causing loudspeaker l_(k) to emit a test signal; -   g) capturing the mobile device microphone signals from said     loudspeaker l_(k) test signal; -   h) determining from said captured mobile device microphone signals a     loudspeaker distance difference value and calculating a     corresponding mobile device position angle value; -   i) calculating a corresponding azimuth ϕ_(k) or polar θ_(k),     respectively, angle value for the position of loudspeaker l_(k); -   j) incrementing k by ‘1’; -   k) if k≤N, returning to step f); -   l) otherwise, checking whether both of ϕ_(k) and θ_(k) have been     determined, and if not true, returning to step b); -   m) after all positions of said N loudspeakers have been determined,     providing a corresponding set of N pairs of azimuth and polar angle     values ϕ_(k) and θ_(k) for said loudspeakers l_(k) and for all k.     The disclosure further pertains first to a measurement device for     determining the direction of loudspeakers l_(k) in a setup of a     surround sound system including N loudspeakers, k=1 . . . N, adapted     to cooperate with a mobile device equipped with at least two     microphones, wherein that direction is expressed by an azimuth angle     ϕ_(k) and a polar angle θ_(k). The mobile device comprises at least     one processor configured for: -   a) setting initial values for the azimuth angle ϕ_(k) and the polar     angle θ_(k) for loudspeaker l_(k) direction; -   b) in a first loop over mobile device position angle α for the     determination of one of ϕ_(k) and θ_(k), and thereafter in a second     loop over mobile device position angle α for the determination of     the other one of ϕ_(k) and θ_(k): -   c) setting k=1; -   d) in a sub-loop over k: -   e) in a sub-sub-loop over a rotation angle of the mobile device: -   f) receiving for the mobile device being rotated a corresponding     measured mobile device rotation angle value α_(k); -   g) receiving corresponding mobile device microphone signals from     emitted loudspeaker l_(k) test signal; -   h) calculating from the microphone signals a corresponding TDOA     value; -   i) if the TDOA value is not zero or is not smaller than a     predetermined threshold value, returning to step f); -   j) otherwise, calculating a corresponding azimuth ϕ_(k) or polar     θ_(k), respectively, angle value for the position of loudspeaker     l_(k); -   k) incrementing k by ‘1’; -   l) if k≤N, returning to step f); -   m) otherwise, checking whether both of ϕ_(k) and θ_(k) have been     determined, and if not true, returning to step b); -   n) after all positions of the N loudspeakers have been determined,     providing a corresponding set of N pairs of azimuth and polar angle     values ϕ_(k) and θ_(k) for the loudspeakers l_(k) and for all k.

In addition, the disclosure pertains secondly to a measurement device for determining ing the direction of loudspeakers l_(k) in a setup of a surround sound system including ing N loudspeakers, k=1 . . . N, adapted to cooperate with a mobile device equipped with at least two microphones, wherein that direction is expressed by an azimuth angle ϕ_(k) and a polar angle θ_(k). The mobile device comprises at least one processor configured for:

-   a) setting initial values for the azimuth angle ϕ_(k) and the polar     angle θ_(k) for loudspeaker l_(k) direction; -   b) in a first loop over mobile device position angle α for the     determination of one of ϕ_(k) and θ_(k), and thereafter in a second     loop over mobile device position angle α for the determination of     the other one of ϕ_(k) and θ_(k), the mobile device having a desired     azimuth angle or polar angle: -   c) setting k=1; -   d) in a sub-loop over k: -   e) receiving mobile device microphone signals from emitted     loudspeaker l_(k) test signal; -   f) determining from said captured mobile device microphone signals a     loudspeaker distance difference value and calculating a     corresponding mobile device position angle value; -   g) calculating a corresponding azimuth ϕ_(k) or polar θ_(k),     respectively, angle value for the position of loudspeaker l_(k); -   h) incrementing k by ‘1’; -   i) if k≤N, returning to step e); -   j) otherwise, checking whether both of ϕ_(k) and θ_(k) have been     determined, and if not true, returning to step b); -   k) after all positions of the N loudspeakers have been determined,     providing a corresponding set of N pairs of azimuth and polar angle     values ϕ_(k) and θ_(k) for the loudspeakers l_(k) and for all k.

Advantageously, the at least one processor is further configured for:

-   a) capturing mobile device microphone signals from loudspeaker l_(k)     test signal emitted by a selected loudspeaker l_(k) among the N     loudspeakers; -   b) receiving for the mobile device a measured mobile device rotation     angle value α_(k) corresponding to a rotation of the mobile device; -   c) calculating a corresponding TDOA value; -   d) if said TDOA value is not zero or is not smaller than a     predetermined threshold value, returning to step a); -   e) otherwise, defining an initial direction angle value β=0; -   f) receiving for the mobile device a measured rotation angle value β     corresponding to rotating the mobile device by an angle β≈π/4; -   g) receiving mobile device microphone signals from emitted     loudspeaker l_(k) test signal; -   h) calculating from the mobile device microphone signals a     loudspeaker distance difference value Δ_(k) and a microphone     distance value

$d_{12} = {\frac{\Delta_{k}}{\sin\;\beta}.}$

BRIEF DESCRIPTION OF DRAWINGS

Exemplary embodiments of the invention are described with reference to the accompanying drawings, which show in:

FIG. 1 spherical coordinate system;

FIG. 2 different time of flight (ToF) for two microphones;

FIG. 3 equal time of flight after re-orientation of microphone pair;

FIG. 4 rotation of microphone pair by angle α_(k) and corresponding (θ_(k), ϕ_(k)),

$\phi_{k} = {\alpha_{k} + \frac{\pi}{2}}$ measured x/y-plane;

FIG. 5 rotation of microphone pair by angle α_(k) and corresponding (θ_(k), ϕ_(k)), θ_(k)=α_(k) measured from the z direction;

FIG. 6 assumption of far-away loudspeaker in relation to the microphone distance;

FIG. 7 ambiguity of loudspeaker location;

FIG. 8 interactive direction measurement for k∈1, . . . , N loudspeakers;

FIG. 9 successive direction measurement for k∈1, . . . , N loudspeakers;

FIG. 10 microphone distance calculation process.

DESCRIPTION OF EMBODIMENTS

Even if not explicitly described, the following embodiments may be employed in any combination or sub-combination.

The correct calibration of a multi-channel audio system requires localisation of multiple speakers. This includes the determination of the direction and distance relative to the listener position. The distance can be measured as described in EP 2899997 A1 or by optical means using the camera of a smartphone. The direction angles are determined by using an acoustical measurement as described below.

Direction Angles

Assuming that the listener position is located in the coordinate origin of a three-dimensional coordinate system, the direction of each loudspeaker can be described by the azimuth angle ϕ and the polar angle θ in spherical coordinates (r, θ, ϕ), see FIG. 1.

The angles (θ, ϕ) can be determined in an interactive way by a device carrying two microphones, or by more devices each carrying one microphone.

Sound Propagation

In the following a microphone pair (m₁, m₂) with known orientation and a speaker l_(k) with unknown position are considered. If the speaker emits a signal s_(k)(t), the signals captured by the microphones will be attenuated and altered by noise. The so-called Time of Flight (ToF) ΔT_(k1) is the time the sound wave needs for propagating from the source (speaker l_(k)) to the microphone m₁. Using a second microphone m₂ the ToF is ΔT_(k2). The signals at the microphone positions are: y _(k1)(t)=g(d _(k1))s _(k)(t−ΔT _(k1))+n ₁(t) y _(k2)(t)=g(d _(k2))s _(k)(t−ΔT _(k2))+n ₂(t)

The function g(d_(k)∘) is an attenuation factor, which describes the dependence of the amplitude on the distance between loudspeaker k and microphone 1 or 2 denoted by d_(k)∘. The amplitudes and the phases of the two signals y_(k1)(t), y_(k2)(t) differ due to the relative positioning of the microphones to the source. The additive terms n₁(t) and n₂(t) take into account environmental and internal (thermal) noise of the microphones.

Angle Determination

The angle measurements can be integrated in a calibration step of a 3D surround sound loudspeaker setup controlled by a smartphone. The determination of the angles are based on the measurement of the Time Difference of Arrival TDOA. The TDOA for loudspeaker l_(k) for the microphone pair (1,2) is defined as τ_(k)=ΔT_(k1)−ΔT_(k2). This corresponds to the spatial difference Δ_(k)=|d_(k1)−d_(k2)|=C|τ_(k)| between the two microphones and the loudspeaker with the sound velocity in air as the scaling factor, see FIG. 2. c is the speed of sound waves in the air.

TDOA Measurement

It is known to estimate the TDOA by using a cross-corre-lation (CC) function R _(k)(τ)=

{y _(k1)(t)y _(k2)(t−τ)}=∫^(+∞) _(−∞) Y _(k1)(f)Y* _(k2)(f)exp^(2πifτ) df with y_(k(1|2))(t) being the signals captured by the microphones (m₁ or m₂ for speaker k) and Y_(k(1|2))(f) being their respective Fourier transforms. The time delay between the captured signals is obtained by searching the peak in the correlation

$\tau_{k} = {\arg\;{\max\limits_{\tau}{{R_{k}(\tau)}.}}}$

Known techniques for providing a sharper peak in the measurement and using interpolation for a higher time resolution can be applied.

Calibration Process—Interactive Angle Measurement

In an interactive measurement a smartphone carrying a pair of microphones is used for the direction determination. It is not necessary that the distance d₁₂ (see FIG. 2) between the microphone pair (m₁, m₂) is known. If the ToF needed for the sound wave to propagate from the source to the first microphone is the same as for the second microphone as is depicted in FIG. 3, the TDOA is zero.

The angles ϕ_(k) and θ_(k) are defined relative to the baseline connecting the two microphones (see FIGS. 4 and 5). In a first step a reference direction is defined from which the angles are measured. For determination of the θ_(k) angle, the microphone pair can be placed in the x/y-plane using the z-axis as reference direction (see FIG. 5).

During playback of the signal from the loudspeaker, the user is moving the smartphone in the direction of the loudspeaker. In this case the TDOA can be continuously measured. This implies an ongoing transmission and capturing of the calibration signal. The device carried by the user can provide a graphical feedback like a level meter which increases if the TDOA is converging to zero. As an alternative, a special sound can be played back if TDOA for the microphones is converging to zero.

In an automatic setting the time delay is measured continuously and the angles yielding the minimal time delay are computed as shown in the FIG. 8 flow chart. The angle measurement is carried out by using corresponding data from the internal sensors of the smartphone.

In step 81, initial values ϕ₀ and θ₀ for the azimuth angle ϕ_(k) and the polar angle θ_(k) are defined, e.g. ϕ₀=θ₀=0. The processing is continued from step 82 to step 88 with a first loop over angle α for the determination of one of ϕ_(k) and θ_(k), e.g. ϕ_(k). Thereafter that loop over angle α is again carried out for the determination of the other one of ϕ_(k) and θ_(k), e.g. θ_(k). In step 89 ϕ_(k) and θ_(k), k=1 . . . N, for all N loudspeaker positions are output.

In step 83 k=1 is set, and within the following sub-loop over k from step 841 to step 87 k is incremented in step 86 until k>N in step 87.

In a sub-sub-loop beginning in step 841, loudspeaker l_(k) emits a test signal s_(k)(t). In step 842 the smartphone is rotated by a recommended angle, e.g. 45° or 90°, and the corresponding true smartphone rotation angle α_(k) is provided from the related sensors within the smartphone. Then the smartphone microphones capture signals y_(k1)(t) and y_(k2)(t) in step 843, and in step 844 τ_(k)(α_(k)) is calculated as described above. By testing step 840 the processing is continued with step 841 for a different smartphone rotation angle, until in step 840 τ_(k)=0 or nearly zero, i.e. until the value τ_(k) is smaller than a predetermined threshold value. If true, in step 85 the corresponding ϕ_(k) or θ_(k), respectively, value is calculated as described above.

Calibration Process—Successive Angle Measurement

In case the distance d₁₂ (see FIG. 2) between the microphone pair (m₁, m₂) is known, e.g. from information taken from a corresponding database, as an alternative to interactive rotation of the smartphone with respect to each loudspeaker for direction determination, another processing can be applied. It can be assumed that the distances d_(k1),d_(k2) between the mobile device and the loudspeakers are much greater than the distance d₁₂ between the microphones, i.e. d_(k1)>>d₁₂. In that case the right-angled triangle in FIG. 6 can be used for the direction computation of N loudspeakers according to smart phone position angle

${\alpha_{k} = {\arcsin\left( \frac{\Delta_{k}}{d_{12}} \right)}},$ k=1, . . . , N.

To avoid the ambiguity about in which half space a loudspeaker is located (see FIG. 7), two successive measurements can be conducted. In the second measurement the device can be rotated by 90°. In this case the determination of the sign of the time delay τ_(k) is sufficient for fixing the direction of the loudspeaker.

In a practical setting each measurement can be conducted for all loudspeakers before performing the next one, as depicted in the FIG. 9 flow chart.

In step 91, initial values ϕ₀ and θ₀ for the azimuth angle ϕ_(k) and the polar angle θ_(k) are defined, e.g. ϕ₀=θ₀=0. The processing is continued from step 92 to step 96 with a first loop over smart phone position angle α for the determination of one of ϕ_(k) and θ_(k), e.g. ϕ_(k). Thereafter that loop over smart phone position angle α is again carried out for the determination of the other one of ϕ_(k) and θ_(k), e.g. θ_(k). In step 97 ϕ_(k) and θ_(k), k=1 . . . N, for all N loudspeaker positions are output.

In step 93 the current position of the smartphone is determined from the internal sensors of the smartphone. In step 94 k=1 is set and, within the following sub-loop processing over k from step 951 to step 950, k is incremented in step 955 until k>N in step 950.

In step 951 loudspeaker l_(k) emits a test signal s_(k)(t). In step 952 the smartphone microphones are capturing signals y_(k1)(t) and y_(k2)(t). Also using d₁₂, in step 953 the loudspeaker distance difference value Δ_(k) and a corresponding smart phone position angle value α_(k) are calculated therefrom as described above, and in step 954 the corresponding ϕ_(k) or θ_(k), respectively, value is calculated as described above.

Calibration Process—Determination of Microphone Distance

In order to conduct a successive measurement as described in the preceding section, a necessary precondition is knowledge of the smartphone microphone distance d₁₂. In case this distance is not known in advance it can be determined by an interactive measurement using one loudspeaker k. During the interactive measurement processing described in connection with FIG. 10, the smartphone is aligned in the direction of the loudspeaker as described in section Interactive angle measurement.

Starting from this reference position, the smartphone is rotated by a predefined angle

$\beta < {\frac{\pi}{2}.}$ In this position the loudspeaker distance difference Δ_(k) is measured and the microphone distance d₁₂ is calculated by

${d_{12} = \frac{\Delta_{k}}{\sin\;\beta}},$ cf. FIG. 6 and FIG. 10.

Microphone distance d₁₂ is then used in the direction determination of the remaining loudspeakers as described in section Successive angle measurement.

In FIG. 10 the calculation process for the microphone distance starts with selecting loudspeaker l_(k) in step 101. In step 1021 that loudspeaker emits a test or playback signal s_(k)(t) and the smartphone is rotated slowly and captures in step 1022 the signals y_(k1)(t) and y_(k2)(t). In step 1023 the current value of τ_(k)(α_(k)) is calculated and in step 1020 it is checked whether the current value of τ_(k) is zero or nearly zero, i.e. is smaller than a predetermined threshold value. If not true, the processing continues with step 1021. If true, the smartphone has reached a desired reference position and the processing moves to step 103 in which an initial direction angle value β=0 is set. In step 104 the smartphone is rotated by β≈π/4 and the corresponding true rotation angle β is provided from the related sensors within the smartphone.

In step 105 loudspeaker l_(k) again emits the test or playback signal s_(k)(t). In step 106 the signals y_(k1)(t) and y_(k2)(t) are captured, and in step 107 the loudspeaker distance difference value Δ_(k) and the microphone distance value d₁₂(β) are calculated.

The described processing can be carried out by a single processor or electronic circuit, or by several processors or electronic circuits operating in parallel and/or operating on different parts of the complete processing.

The instructions for operating the processor or the processors according to the described processing can be stored in one or more memories. The at least one processor is configured to carry out these instructions. 

The invention claimed is:
 1. A method for using a smartphone equipped with at least two microphones (m₁,m₂) for determining the direction of loudspeakers A, in a setup of a surround sound system including N loudspeakers, k=1 . . . N, wherein said direction is expressed by an azimuth angle ϕ_(k) and a polar angle θ_(k), said method including: a) setting initial values (ϕ₀,θ₀) for said azimuth angle ϕ_(k) and said polar angle θ_(k) for loudspeaker l_(k) direction; b) in a first loop over smartphone position angle α for the determination of one of ϕ_(k) and θ_(k), and thereafter in a second loop over smartphone position angle α for the determination of the other one of ϕ_(k) and θ_(k): c) setting k=1; d) in a sub-loop over k: e) in a sub-sub-loop over a rotation angle of said smartphone: f) causing loudspeaker l_(k) to emit a test signal (s_(k)(t)); g) rotating said smartphone and providing for said smartphone a corresponding measured smartphone rotation angle value α_(k), h) capturing corresponding smartphone microphone signals (y_(k1)(t), y_(k2)(t)) from said loudspeaker l_(k) test signal; i) calculating from said microphone signals a corresponding Time Difference of Arrival value (τ_(k)(α_(k))); j) if said Time Difference of Arrival value (τ_(k)(α_(k))) is not zero or is not smaller than a predetermined threshold value, returning to step f); k) otherwise, calculating a corresponding azimuth ϕ_(k) or polar θ_(k), respectively, angle value for the position of loudspeaker l_(k); L) incrementing k by ‘1’; m) if k≤N, returning to step f); n) otherwise, checking whether both of ϕ_(k) and θ_(k) have been determined, and if not true, returning to step b); o) after all positions of said N loudspeakers have been determined, providing a corresponding set of N pairs of azimuth and polar angle values ϕ_(k) and θ_(k) for said loudspeakers l_(k) and for all k; p) using said corresponding set of pairs of azimuth and polar angle values to accurately calibrate said loudspeakers l_(k).
 2. The method for using a smartphone equipped with at least two microphones (m₁,m₂), having a known distance (d₁₂) from each other, for determining the direction of loudspeakers l_(k) in a setup of a surround sound system including N loudspeakers, k=1 . . . N, wherein said direction is expressed by an azimuth angle ϕ_(k) and a polar angle θ_(k), said method including: a) setting initial values (ϕ₀,θ₀) for said azimuth angle ϕ_(k) and said polar angle θ_(k) for loudspeaker l_(k) direction; b) in a first loop over smartphone position angle α for the determination of one of ϕ_(k) and θ_(k), and thereafter in a second loop over smartphone position angle α for the determination of the other one of ϕ_(k) and θ_(k): c) positioning said smartphone at a desired azimuth angle or polar angle; d) setting k=1; e) in a sub-loop over k: f) causing loudspeaker l_(k) to emit a test signal (s_(k)(t)); g) capturing the smartphone microphone signals (y_(k1)(t), y_(k2)(t)) from said loudspeaker l_(k) test signal; h) determining from said captured smartphone microphone signals (y_(k1)(t), y_(k2)(t)) a loudspeaker distance difference value (Δ_(k)) and calculating a corresponding smartphone position angle value (α_(k)): i) calculating a corresponding azimuth ϕ_(k) or polar θ_(k), respectively, angle value for the position of loudspeaker l_(k); j) incrementing k by ‘1’; k) if k≤N, returning to step f); I) otherwise, checking whether both of ϕ_(k) and θ_(k), have been determined, and if not true, returning to step b); m) after all positions of said N loudspeakers have been determined, providing a corresponding set of N pairs of azimuth and polar angle values ϕ_(k) and θ_(k) for said loudspeakers l_(k) and for all k; n) using said corresponding set of pairs of azimuth and polar angle values to accurately calibrate said loudspeakers l_(k).
 3. The method according to claim 2, wherein for determining the distance (d₁₂) between said two microphones (m₁, m₂) the following processing is carried out: a) selecting one loudspeaker l_(k) of said N loudspeakers; b) causing loudspeaker l_(k) to emit a test signal (s_(k)(t)); c) capturing the smartphone microphone signals (y_(k1)(t), y_(k2)(t)) from said loudspeaker l_(k) test signal; d) rotating said smartphone and providing for said smartphone a corresponding measured smartphone rotation angle value α_(k); e) calculating a corresponding Time Difference of Arrival value (τ_(k)(α_(k))); f) if said Time Difference of Arrival value (τ_(k)(α_(k))) is not zero or is not smaller than a predetermined threshold value, returning to step b); g) otherwise, defining an initial direction angle value β=0; h) rotating said smartphone by an angle β≈π/4 and providing for said smartphone a corresponding measured rotation angle value β; i) causing loudspeaker l_(k) to emit a test signal (s_(k)(t)); j) capturing the smartphone microphone signals (y_(k1)(t), y_(k2)(t)) from said loudspeaker l_(k) test signal; k) calculating from said smartphone microphone signals (y_(k1)(t),y_(k2)(t)) a loudspeaker distance difference value Δ_(k) and a microphone distance value $d_{12} = {\frac{\Delta_{k}}{\sin\;\beta}.}$
 4. The method according to claim 1, wherein said smartphone includes an app that controls the processing.
 5. The method according to claim 2, wherein smartphone includes an app that controls the processing.
 6. The method according to claim 1, wherein said smartphone microphone signals are y _(k1)(t)=g(d _(k1))s _(k)(t−ΔT _(k1))+n ₁(t) and y _(k2)(t)=g(d _(k2))s _(k)(t−ΔT _(k2))+n ₂(t), wherein ΔT_(k1) is the time the sound wave needs for propagating from loudspeaker l_(k) to microphone m₁ and ΔT_(k2) is the time the sound wave needs for propagating from loudspeaker l_(k) to microphone m₂, S_(k)(∘) is said test signal, g(d_(k∘)) is an attenuation factor which describes the dependence of the amplitude on the distance d_(k∘) between loudspeaker l_(k) and microphone m₁ or m₂, and n₁(t) and n₂(t) take into account environmental and internal noise of said microphones.
 7. The method according to claim 2, wherein said smartphone microphone signals are y _(k1)(t)=g(d _(k1))s _(k)(t−ΔT _(k1))+n ₁(t) and y _(k2)(t)=g(d _(k2))s _(k)(t−ΔT _(k2))+n ₂(t), wherein ΔT_(k1) is the time the sound wave needs for propagating from loudspeaker l_(k) to microphone m₁ and ΔT_(k2) is the time the sound wave needs for propagating from loudspeaker l_(k) to microphone m₂, S_(k)(∘) is said test signal, g(d_(k∘)) is an attenuation factor which describes the dependence of the amplitude on the distance d_(k∘) between loudspeaker l_(k) and microphone m₁ or m₂, and n₁(t) and n₂(t) take into account environmental and internal noise of said microphones.
 8. The method according to claim 6, wherein said Time Difference of Arrival for loudspeaker l_(k) for said smartphone microphones is defined as τ_(k)=ΔT_(k1)−τ_(k2), which corresponds to the spatial difference Δ_(k)=|d_(k1)−d_(k2)|=c|τ_(k)| between said smartphone microphones and said loudspeaker l_(k) with the sound velocity c in air as a scaling factor.
 9. The method according to claim 3, wherein said Time Difference of Arrival for loudspeaker l_(k) for said smartphone microphones is defined as τ_(k)=ΔT_(k1)−τ_(k2), which corresponds to the spatial difference Δ_(k)=|d_(k1)−d_(k2)|=c|τ_(k)| between said smartphone microphones and said loudspeaker l_(k) with the sound velocity c in air as a scaling factor.
 10. The method according to claim 1, wherein said Time Difference of Arrival is estimated by using a cross-correlation function R _(k)(τ)=

{y _(k1)(t)y _(k2)(t−τ)}=∫_(−∞) ^(+∞) Y _(k1)(f)Y _(k2)*(f)exp^(2πifτ) df with y_(k(1|2))(t) being the signals captured by said smartphone microphones and y_(k)(1l₂)(f) being their respective Fourier transforms, and wherein the time delay between the microphone signals is obtained by searching the peak in the correlation $\tau_{k} = {\arg{\max\limits_{\tau}{{R_{k}(\tau)}.}}}$
 11. The method according to claim 3, wherein said Time Difference of Arrival is estimated by using a cross-correlation function R _(k)(τ)=

{y _(k1)(t)y _(k2)(t−τ)}=∫_(−∞) ^(+∞) Y _(k1)(f)Y _(k2)*(f)exp^(2πifτ) df with y_(k(1|2))(t) being the signals captured by said smartphone microphones and y_(k(1|2))(f) being their respective Fourier transforms, and wherein the time delay between the microphone signals is obtained by searching the peak in the correlation $\tau_{k} = {\arg{\max\limits_{\tau}{{R_{k}(\tau)}.}}}$
 12. The method according to claim 1, wherein, instead of interactive rotation of said smartphone with respect to each loudspeaker for direction determination, it is assumed that the distances d_(k1), d_(k2) between the microphones of said smartphone and said loudspeaker are much greater than the distance d₁₂ between the microphones in said smartphone, and the angle α_(k) between the line between both microphones and the direction of said loudspeaker is ${\alpha_{k} = {\arcsin\left( \frac{\Delta_{k}}{d_{12}} \right)}},$ k=1, . . . , N, and wherein, in order to avoid the ambiguity about in which half space a loudspeaker is located, two successive measurements are conducted and in the second measurement said smartphone is rotated by approximately 90° and the determination of the sign of said time delay τ_(k) is used for fixing the direction of said loudspeaker.
 13. A computer program product stored on a non-transitory computer readable medium comprising instructions which, when carried out on a mobile device, perform the method according to claim
 1. 14. A computer program product stored on a non-transitory computer readable medium comprising instructions which, when carried out on a mobile device, perform the method according to claim
 2. 15. A measurement device for determining the direction of loudspeakers l_(k) in a setup of a surround sound system including N loudspeakers, k=1 . . . N, adapted to cooperate with a smartphone equipped with at least two microphones (m₁,m₂), wherein said direction is expressed by an azimuth angle ϕ_(k) and a polar angle θ_(k), said smartphone comprising at least one processor configured for: a) setting initial values (ϕ₀,θ₀) for said azimuth angle ϕ_(k) and said polar angle θ_(k) for loudspeaker l_(k) direction; b) in a first loop over smartphone position angle α for the determination of one of ϕ_(k) and θ_(k), and thereafter in a second loop over mobile device position angle α for the determination of the other one of ϕ_(k) and θ_(k): c) setting k=1; d) in a sub-loop over k: e) in a sub-sub-loop over a rotation angle of said smartphone; f) receiving for said smartphone being rotated a corresponding measured smartphone rotation angle value α_(k); g) receiving corresponding smartphone microphone signals (y_(k1)(t), y_(k2)(t)) from emitted loudspeaker l_(k) test signal; h) calculating from said microphone signals a corresponding Time Difference of Arrival value (τ_(k)(α_(k))); i) if said Time Difference of Arrival value (τ_(k)(α_(k))) is not zero or is not smaller than a predetermined threshold value, returning to step f); j) otherwise, calculating a corresponding azimuth ϕ_(k) or polar θ_(k), respectively, angle value for the position of loudspeaker l_(k); k) incrementing k by ‘1’; I) if k≤N, returning to step f); m) otherwise, checking whether both of ϕ_(k) and θ_(k) have been determined, and if not true, returning to step b); n) after all positions of said N loudspeakers have been determined, providing a corresponding set of N pairs of azimuth and polar angle values ϕ_(k) and θ_(k) for said loudspeakers l_(k) and for all k; o) using said corresponding set of pairs of azimuth and polar angle values to accurately calibrate said loudspeakers l_(k).
 16. A measurement device for determining the direction of loudspeakers l_(k) in a setup of a surround sound system including N loudspeakers, k=1 . . . N, adapted to cooperate with a smartphone equipped with at least two microphones (m₁,m₂), wherein said direction is expressed by an azimuth angle ϕ_(k) and a polar angle θ_(k), said smartphone comprising at least one processor configured for: a) setting initial values (ϕ₀,θ₀) for said azimuth angle ϕ_(k) and said polar angle θ_(k) for loudspeaker l_(k) direction; b) in a first loop over position angle α for the determination of one of ϕ_(k) and θ_(k), and thereafter in a second loop over smartphone position angle α for the determination of the other one of ϕ_(k) and θ_(k), said smartphone having a desired azimuth angle or polar angle: c) setting k=1; d) in a sub-loop over k: e) receiving smartphone microphone signals (y_(k1)(t), y_(k2)(t)) from emitted loudspeaker l_(k) test signal (s_(k)(t)); f) determining from said captured smartphone microphone signals (y_(k1)(f), y_(k2)(t)) a loudspeaker distance difference value (Δ_(k)) and calculating a corresponding smartphone position angle value (α_(k)); g) calculating a corresponding azimuth ϕ_(k) or polar θ_(k), respectively, angle value for the position of loudspeaker l_(k); h) incrementing k by ‘1’; i) if k≤N, returning to step e); j) otherwise, checking whether both of ϕ_(k) and θ_(k) have been determined, and if not true, returning to step b); k) after all positions of said N loudspeakers have been determined, providing a corresponding set of N pairs of azimuth and polar angle values ϕ_(k) and θ_(k) for said loudspeakers l_(k) and for all k; l) using said corresponding set of pairs of azimuth and polar angle values to accurately calibrate said loudspeakers l_(k).
 17. The measurement device of claim 16, in which said at least one processor is further configured for: a) capturing smartphone microphone signals (y_(k1)(t), y_(k2)(t)) from loudspeaker l_(k) test signal emitted by a selected loudspeaker l_(k) among said N loudspeakers; b) receiving for said smartphone a measured smartphone rotation angle value α_(k) corresponding to a rotation of said smartphone; c) calculating a corresponding Time Difference of Arrival value (τ_(k)(α_(k))); d) if said Time Difference of Arrival value (τ_(k)(α_(k))) is not zero or is not smaller than a predetermined threshold value, returning to step a); e) otherwise, defining an initial direction angle value β=0; f) receiving for said smartphone a measured rotation angle value β corresponding to rotating said smartphone by an angle β≈π/4; g) receiving smartphone microphone signals from emitted loudspeaker l_(k) test signal; h) calculating from said smartphone microphone signals a loudspeaker distance difference value Δ_(k) and a microphone distance value $d_{12} = {\frac{\Delta_{k}}{\sin\;\beta}.}$ 